1. Field of the Invention
This invention relates to improvements in electrical circuits, and more particularly to improvements in electrical circuity of the type used in estimating average current flowing in a load, and still more particularly, in one preferred embodiment, to improvements in electrical circuitry of the type used to estimate or reconstruct average current flowing in a polyphase dc motor, both in linear and pulse width modulation modes of motor operation.
2. DESCRIPTION OF THE PRIOR ART
One widely used method of sensing a current flowing in a load is to place a sensing element, such as a resistor, in a series current path between a supply voltage and the load, and measuring the voltage across it. The current can then be directly calculated by dividing the measured voltage by the value of the resistor. In some applications multiple loads exist, for example, polyphase dc motor coils, or the like, to which drive signals are switchably connected in a predetermined sequence, and for which it is necessary to measure the current flowing in the coils In typical circuits for providing drive signals to such loads, the sense resistor is usually placed between a reference potential, such as V.sub.cc and ground, or the switches that connect the drive current to the selected coils of the motor in accordance with the switched commutation sequence. Such schemes generally improve the common mode rejection ratio and help to implement transconductance control loops.
By placing a single sense resistor between the current switches and ground in transconductance circuits for driving the motor coils, only a single sensing element is required to convey instantaneous current information for any energized coil load. The major shortcomings of such a circuit arrangement emerge when pulse width modulation (PWM) systems are used in conjunction with the motor driver. Pulse width modulation techniques, often used to reduce the power dissipation in polyphase motors, chop the current in the coils of the motor at their peak current levels to achieve maximum torque, to allow rapid accelerations, and to reduce the power dissipated in the chip to a level proportional to the duty cycle. Typically, during this chopping, the transistor switches that control the current to the instantaneously active coils of the motor are switched on and off by a network of switching transistors.
In order for the motor to be able to be smoothly switched between the linear and PWM modes without transients appearing in the speed of the motor, it is helpful to know the value of the current flowing in the motor coils. However, during the PWM mode of operation, information which would indicate the current flowing in the motor (i.e., the voltage on the sense resistor) is present only during the "ON" time of the switches in the various current paths. When the switches are "OFF" in the PWM mode, the voltage on the sense resistor does not accurately represent the current which continues to flow in the inductive coil loads.
Several schemes have been proposed to address this problem, most requiring complicated circuit arrangements, such as complicated diode steering arrangements that insure a load current always flows through the sense resistor. Such previously proposed arrangements, however, usually require at least some energy dissipation, and generally can not be practically implemented in D-MOS switch type circuits because of the intrinsic diode associated with the typical D-MOS switching transistors used in most motor switching designs.